a) Field of the Invention
The present invention relates to an optical system for endoscopes to be use in medical and industrial fields, and more specifically to an optical system for endoscopes which is composed of an objective lens system and relay lens systems.
b) Description of the Prior Art
As an example of the conventional retrofocus type objective lens systems for endoscopes, there is known the objective lens system proposed by Japanese Patent Kokai Publication No. Sho 59-226315.
This conventional example is composed, as illustrated in FIG. 1, a first lens unit L.sub.1 having a negative refractive power, an aperture stop S and a second lens unit L.sub.2 having a positive refractive power which are arranged in order from the object side. The first lens unit L.sub.1 has a negative refractive power for enlarging the visual field angle of the objective lens system and the second lens unit L.sub.2 has an imaging function so as to compose a telecentric objective lens system which has a large field angle, the characteristic of an objective lens system for endoscopes, and allows the principal ray P to be incident perpendicularly on the image surface thereof at any image height.
Such an objective lens system for endoscopes 5 must be telecentric for preventing images transmitted through the relay lens systems from being darkened as described below: When a relay lens system which has an entrance pupil at infinite distance is arranged on the image surface of the objective lens system, rays are allowed to be incident on the relay lens system only within a certain range of angles and, if the principal ray P is incident obliquely on the image surface, transmission efficiency of light is lowered, thereby darkening an image transmitted through the relay lens system. Description will be made on a concrete optical system which is composed of the objective lens system shown in FIG. 1 and relay lens systems as illustrated in FIG. 2. An image O.sub.2 formed by the objective lens system is transmitted by relay lens systems R.sub.1, R.sub.2 and R.sub.3 as images O.sub.2, O.sub.3 and O.sub.4, and at the same time, pupils which determine brightness of the images are also transmitted. These pupils are located at the position S in the objective lens system and the positions S.sub.1, S.sub.2, S.sub.3, in the relay lens systems, and sizes of the pupils are determined, in most cases, nearly by outside diameters of the relay lens systems. Therefore, it is not always necessary to arrange an aperture stop in the objective lens system.
Further, the case of an objective lens system for endoscopes comprising a visual field changing prism P.sub.1, it is difficult to arrange an aperture stop S in this objective lens system due to the fact that the visual field direction changing prism P: is arranged in the vicinity of the location at which the aperture stop S is to be arranged. It is therefore convenient to combine relay lens systems with the objective lens system which hardly allows arrangement of an aperture stop therein with relay lens systems.
As a conventional example of an optical system composed of an objective lens system for endoscopes and relay lens systems, there is known the optical system disclosed by Japanese Patent Kokai Publication No. Hei 3-39915. This optical system adopts a retrofocus type objective lens system comprising a lens component L.sub.1 having an aspherical surface disposed thereon and a negative refractive power and a lens unit L.sub.2 having a positive refractive power as illustrated in FIG. 4. This objective lens system, which is its same in the fundamental composition as the objective lens system disclosed by Japanese Patent Kokai Publication No. Sho 59-226315, uses an aspherical surface ASP in a first lens unit L.sub.1 which is designed to satisfy a relation expressed by the following formula (i): EQU I=f tan .theta..sub.1 (i)
wherein the reference symbol I represents image height, the reference symbol f designates the focal length of the objective lens system and the reference symbol .theta..sub.1 denotes the angle formed between the optical axis and the ray incident on the aspherical surface.
Further, said objective lens system comprises a second lens unit L.sub.2 which is designed to satisfy a relation expressed by the following formula (ii): EQU I=f.sub.2 sin .theta..sub.2 (ii)
wherein the reference symbol f.sub.2 represents the focal length of the second lens unit L.sub.2 and the reference symbol .theta..sub.2 designates the angle formed between the optical axis and the ray incident on the object side surface of the second lens unit L.sub.2.
The retrofocus type objective lens system for endoscopes which was known before the objective lens system described above was designed to satisfy the sine condition I=f sin .theta..sub.1, whereas the above-described objective lens system satisfies the condition I=f tan .theta..sub.1 by using the aspherical surface for correcting the distortion which is aggravated abruptly as visual field angle .omega. is widened. Further, the second lens unit L.sub.2 of the above-described objective lens system is designed to satisfy the sine condition for maintaining uniform brightness over the entire range of an image from the center to the marginal portion thereof.
The objective lens system disclosed by above-mentioned Japanese Patent Kokai Publication No. Hei 3-39915 can have corrected distortion and a widened visual field angle. When the visual field angle of this objective lens system is further widened, however, a large angle is formed between the optical axis and the principal ray in a light bundle travelling from the aspherical surface toward the object side. When a cover glass plate CG is arranged on the object side of the aspherical surface ASP as shown in FIG. 4, for example, a ray having a high image height passes through a portion of the cover glass plate CG which is far from the optical axis and the cover glass plate CG must have a large outside diameter larger than that of the objective lens system which is restricted for use with endoscopes, thereby making the objective lens system unsuited for use with endoscopes.
As a means for bringing the ray passing through the cover glass plate CG nearer the optical axis, it is conceivable to displace the location of the aperture stop S nearer the cover glass plate CG as shown in FIG. 5. When the aperture stop S is brought nearer the cover glass plate CG, however, a ray having a high image height passes through the aspherical surface ASP of the first lens unit L.sub.1 at a portion which is also brought nearer the optical axis and PG,7 it is necessary for satisfying the relationship of I=f tan .theta..sub.1 to design the aspherical surface so as to have a shape which abruptly varies as the portions of said aspherical surface are farther from the optical axis. It is practically very difficult to form an aspherical surface of such a shape on a lens element having a small outside diameter.
FIG. 6 exemplifies concrete shapes of aspherical surfaces which are to be used for designing objective lens systems which have focal lengths on the order of 1 mm, visual field angle .omega. of 43.2.degree. (2.omega.=86.4.degree.) and distortion DT of different amounts. As is understood from this drawing, the aspherical surface must have shapes which are in practice difficult to form for obtaining distortion DT of -10% to -15% in the objective lens system.